Although it may appear that way, I don't think you have to sign up.https://www.streetlife.com/area/hertford/
I agree that they seem to be somewhat opaque about who they are. There's no contact or ownership details - the contact link just takes you to a support page.
A little digging will reveal their Press Contact pagehttps://www.streetlife.com/about/contact/
And their "team" can be found athttps://www.streetlife.com/about/team/
I understand they're rolling out county by county.
Archant pumped £600,000 into them back in July 2013.
It was originally set up by a guy in Battersea who didn't have the gumption to knock on his neighbours' door after he was burgled and assumes that all communities are devoid of neighbourliness and couldn't possibly have thought up anything as clever as a local online community forum or Facebook page.